1. Field of the Invention
The present invention relates to the ammoxidation of saturated hydrocarbons, and, more especially, to the ammoxidation/conversion of alkanes into a mixture containing .alpha.,.beta.-unsaturated nitriles.
2. Description of the Prior Art
The ammoxidation of olefins and, in particular, of propylene, is well known to this art. However, although the saturated hydrocarbons, which are more widely available, are the more desirable starting materials from an economic standpoint, it is also well known to this art that these do not display comparable reactivity in this type of reaction to form, especially, .alpha.,.beta.-unsaturated nitriles.
One of the difficulties encountered in the ammoxidation of saturated hydrocarbons is the requirement for catalysts capable of dehydrogenating the saturated hydrocarbon under conditions which minimize or eliminate the combustion of the ammonia and/or that of the hydrocarbon, while at the same time ensuring a reasonable selectivity either for the .alpha.,.beta.-unsaturated nitrile (target compound), for example for acrylonitrile starting from propane, or for added value compounds (above-mentioned nitrile and olefin), for example, for acrylonitrile and propylene starting from propane.
U.S. Pat. No. 3,365,482, describes the ammoxidation, especially of isobutane into methacrylonitrile, on a molybdenum-based catalyst deposited onto eta-alumina, doped with antimony, at 508.degree. C., beginning with a gaseous mixture containing isobutane, air, ammonia and steam (1.0/4.5/1.0/12.5); the selectivity for methacrylonitrile attains a value of 49% for a degree of conversion of the isobutane of 22%.
When the starting material is a gaseous mixture of propane/air/ammonia/steam (1.0/4.7/0.67/12.8), using the same catalyst and at 550.degree. C., the selectivity for acrylonitrile decreases to 15% for a degree of conversion of the propane of 29%.
In Chemistry Letters, pp. 2173-2176 (1989), the ammoxidation of propane in the vapor phase is described, in the presence of multicomponent metal oxides containing molybdenum and bismuth and having a structure of the type of that of scheelite. It appears that, despite the relatively moderate temperatures used, the proportion of combustion products (CO, CO.sub.2) is very high in all instances (at least 15%) and that certain catalytic compositions tested exhibit very little activity with respect to the desired reaction, while being used under conditions which are in the explosive region or very near the explosive region.
It is immediately apparent that the coproduction of large amounts of CO and CO.sub.2 is undesirable on an industrial scale.
In addition, the use of reaction mixtures which are in the explosive region, compositionally, is even less desirable on an industrial scale, since the process is carried out in a stationary bed.
U.S. Pat. No. 4,760,159 describes a process for the ammoxidation of an alkane having from 3 to 5 carbon atoms into an .alpha.,.beta.-unsaturated nitrile having from 3 to 5 carbon atoms, by reaction in the gas phase with ammonia and oxygen, in the presence of a solid catalyst having the formula Bi.sub.a V.sub.b L.sub.l M.sub.m T.sub.t O.sub.x, in which Bi, V, M (which is selected from among Mo, W, Cr, Ge and Sb) and 0 are necessarily present.
From the various examples set forth in this '159 patent, it is seen that the best selectivities towards .alpha.,.beta.-unsaturated nitriles are obtained using catalysts containing vanadium, bismuth and molybdenum and, optionally, another cation such as chromium, potassium, zinc, cesium or antimony. The only example using a catalyst based on vanadium, bismuth and antimony, having the formula BiV.sub.0.7 Sb.sub.0.50 O.sub.x and deposited onto a silica/alumina mixture, in an amount of 50% by weight, produces yields of .alpha.,.beta.-unsaturated nitrile which are markedly lower than those attained using a catalyst containing molybdenum.
Additionally, this process also forms a not insignificant amount of the oxides of carbon (CO+CO.sub.2).